Embodiments of the present invention relate to a process for the preparation of copper containing molecular sieves with the CHA structure having a silica to alumina mole ratio greater than about 10, wherein the copper is exchanged into the Na+-form of the Chabazite, using a liquid copper solution wherein the concentration of copper is in the range of about 0.001 to about 0.4 molar. In addition, this invention relates to copper containing molecular sieves with the CHA structure, obtainable or obtained by the above-described process, and catalysts, systems and methods.
Both synthetic and natural zeolites and their use in promoting certain reactions, including the selective catalytic reduction (SCR) of nitrogen oxides with a reductant like ammonia, urea and/or hydrocarbon in the presence of oxygen, are well known in the art. Zeolites are aluminosilicate crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms in diameter. Chabazite (CHA) is a small pore zeolite with 8 member-ring pore openings (˜3.8 Angstroms) accessible through its 3-dimensional porosity (as defined by the International Zeolite Association). A cage like structure results from the connection of double six-ring building units by 4 rings.
X-ray diffraction studies on cation locations in Chabazite have identified seven cation sites coordinating with framework oxygens, labeled A, B, C, D, F, H, and I. They are located in the center of double six-membered ring, on or near the center of the six-membered ring in Chabazite cage, and around the eight-membered ring of the chabazite cage, respectively. The C site is located slightly above the six-membered ring in the Chabazite cage and the F, H and I sites are located around the eight-membered ring in the Chabazite cage (see Mortier, W. J. “Compilation of Extra Framework Sites in Zeolites”, Butterworth Scientific Limited, 1982, p 11 and Pluth, J. J., Smith, J. V., Mortier, W. J., Mat. Res. Bull., 12 (1977) 1001).
The catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, 200° C. to 600° C. or higher, under hydrothermal conditions. Hydrothermal conditions are often encountered in practice, such as during the regeneration of a soot filter, a component of the exhaust gas treatment system used for the removal of particles.
Metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, for the selective catalytic reduction of nitrogen oxides with ammonia are known. Iron-promoted zeolite beta (U.S. Pat. No. 4,961,917) has been an effective commercial catalyst for the selective reduction of nitrogen oxides with ammonia. Unfortunately, it has been found that under harsh hydrothermal conditions, for example exhibited during the regeneration of a soot filter with temperatures locally exceeding 700° C., the activity of many metal-promoted zeolites begins to decline. This decline is often attributed to dealumination of the zeolite and the consequent loss of metal-containing active centers within the zeolite.
The process of preparation of metal containing Chabazite as known in the art can be divided in four sub-steps i) crystallization of the organic template containing Na-Chabazite, ii) calcination of Na-Chabazite, iii) NH4-exchange to form NH4-Chabazite and iv) metal-exchange into to NH4-Chabazite to form metal-Chabazite. The NH4-exchange step aims to remove alkali metals (e.g. Na) which are detrimental to the hydrothermal stability of the final catalyst.
The typical Na2O level of Na-Chabazite is between 6000 and 8000 ppm. Sodium is known to degrade the zeolite structure under hydrothermal aging conditions via formation of Na4SiO4 and Na2Al2O4 and concomitant dealumination of the zeolite. In order to keep the Na2O content low, an NH4-exchange with for example NH4NO3 is carried out in a third step.
Dedecek et al. describes in Microporous and Mesoporous Materials 32 (1999) 63-74 a direct copper exchange into Na+-, Ca2+-, Cs+-, Ba2+-form of Chabazite. An aqueous solution of copper acetate is used with copper concentrations varying between 0.20 and 7.6 wt % that is between 0.001 and 0.1 molar. The liquid to solid ratio varies between 20 and 110. The silica to alumina ratio is between 5 and 8. In all direct exchanges (i.e. copper in to the Na-form of the zeolite) of the natural chabazite, the total alkali metal content of the copper containing molecular sieves with the CHA structure is greater than about 4.6 wt % (expressed as the metal oxide). Additionally, in the direct exchange of synthetic Na-Chabazite, the sodium content is greater than about 0.97 wt % Na2O when one exchange step is used, or about 0.73 wt % Na2O when 2 exchange steps are used.
WO 2008/77590 describes a process of direct metal exchange into Na+-form of a zeolite material, wherein the metal-exchange is done by suspending a zeolite material in an aqueous solution which comprises metal ions and ammonium ions. As specific non-limiting examples of metal ions, iron, silver, and copper are described. The use of ammonium double salt is used in specific embodiments. In the examples BEA was used as zeolite material and ammonium iron(II) sulfate hexahydrate as iron source having a concentration of about 0.025 and 0.09 molar. No catalytic data are disclosed.
The technical challenge of the direct copper exchange process is to replace the residual Na+ ions with Cu2+ ions and reach target loadings of both metals to simultaneously meet catalytic performance and stability needs of the SCR process. Both excess CuO and residual Na2O are assumed to have a detrimental effect on the catalyst performance after aging.
WO 2008/106519 discloses a catalyst comprising: a zeolite having the CHA crystal structure and a mole ratio of silica to alumina greater than 15 and an atomic ratio of copper to aluminum exceeding 0.25. The catalyst is prepared via copper exchanging NH4+-form CHA with copper sulfate or copper acetate. The copper concentration of the aqueous copper sulfate ion-exchange step varies from 0.025 to 1 molar, where multiple copper ion-exchange steps are needed to attain target copper loadings. The catalyst resulting from copper sulfate ion-exchange exhibits NOx conversion from 45 to 59% at 200° C. and ˜82% at 450° C. Free copper must be added to improve the 200° C. performance after aging. 0.4 M copper acetate exchange results in a material with NOx conversion after aging of 70 and 88% at 200 and 450° C., respectively. In WO 2008/106519 a large excess of copper is used in order to reach a CuO loading of about 3 wt %; the typical Cu exchange yield using copper sulfate is only about 4%. For copper acetate, the Cu exchange yield is between 24 and 31%.
US 2008/0241060 and WO 2008/132452 disclose that zeolite material can be loaded with iron and/or copper, whereas iron and/or copper are introduced into the mircoporous crystalline material by aqueous or solid state ion-exchange or incorporated by a direct-synthesis (during zeolite synthesis), whereas a direct-synthesis does not require a metal doping process after the zeolite has been formed. In the examples of US 2008/0241060, NH4NO3 was used to remove residual sodium, but the copper ion-exchange is not described. Example 2 of WO 2008/132452 states that an ammonium exchange was carried out before an aqueous copper exchange using copper nitrate. It is stated that multiple aqueous ion-exchanges were carried out to target 3 wt % Cu. No details of reaction conditions were provided.
There is an on-going desire to simplify the process of preparing copper containing molecular sieves with the CHA structure as this process contains many processing steps adding capital and operating cost to the manufacturing process.